The present invention disclosed herein relates to an organic light emitting device, and more particularly, to an organic light emitting device having high light extraction efficiency, lowering leakage current, and improving reliability.
An organic light emitting device, for example, an organic light emitting diode is a light emitting device, where excitons occur when holes supplied from an anode electrode and electrons supplied from a cathode electrode are combined in an organic light emitting layer therebetween and they are recombined again. The organic light emitting diode as a self-light-emitting device is applied to a display device and developed due to its wide viewing angle, fast response speed, and high color reproduction range. Furthermore, recent research and development on applying an organic light emitting diode are actively in progress. The organic light emitting diode may be configured to emit R (red), G (green), and B (blue) separately, or to emit white color.
According to Thompson, external quantum efficiency that represents the luminous efficiency of an organic light emitting device may be represented with the multiplication of the internal quantum efficiency and light extraction efficiency of a device, and may not exceed about 20% because the light emitted from an organic light emitting layer may not be emitted to a substrate external due to total reflection at the interface between layers having a different refractive index and accordingly thereto may be trapped in each layer inside (Optics Letters 22, 6, 396, 1997). That is, the external quantum efficiency of an organic light emitting device does not surpass about 20%. The light trapped in a layer of an organic light emitting diode is called guided mode light, and the light emitted to external air, passing through the interface of each layer, is called an out-coupling mode light. In a plane light source having a panel shape, converting a guided mode light into an out-coupling mode light is called light extraction.
In order to improve light extraction efficiency, materials, which have the same or higher refractive index as it approaches from an organic light emitting device toward a light emitting direction, may be stacked. However, since a transparent substrate used for organic light emitting devices, for example, a glass substrate, has a low refractive index of about 1.5, some limitations in the light extraction may occur.
FIG. 1 is a schematic view illustrating a layer stacked structure of a typical organic light emitting diode. The organic light emitting diode includes a sequentially stacked substrate 10, anode (i.e. a transparent electrode) 20, organic light emitting layer 30, cathode (i.e. a reflective electrode) 40, and protective layer 50.
In the typical organic light emitting diode, among light generated from the organic light emitting layer 30, the light emitted toward a cathode direction is mostly reflected so that it is emitted toward an anode direction. As a result, most of the generated light is emitted toward the anode side. At this point, in an organic light emitting diode having an anode on a substrate, in order to emit light, a transparent substrate such as a glass substrate is used. When light passes through an organic light emitting layer, an anode, and substrate to be emitted to air, due to a refractive index difference of each layer, a reflective light {circle around (1)} between an organic light emitting layer and an anode layer, a reflective light {circle around (2)} between an anode layer and a substrate, and a reflective light {circle around (3)} between the substrate and air occur. Especially, by Snell's law below (i.e. Equation 1), lights incident into the interface from a high refractive index medium into a low refractive index medium at an angle grater than a critical angle perpendicular to a substrate surface are all reflected so that they are not emitted to an external and extinct in a device internal.
                                          n            1                                n            2                          =                              sin            ⁢                                                  ⁢                          a              2                                            sin            ⁢                                                  ⁢                          a              1                                                          [                  Equation          ⁢                                          ⁢          1                ]            where n1 is a refractive index of a material before incident, n2 is a refractive index of a material after incident, a1 is an incident angle with respect to the normal of an incident surface, and a2 is a refraction angle with respect to the normal of an incident surface.
A refractive index of an organic light emitting layer in an organic light emitting diode may vary according to wavelengths of light and generally ranges from about 1.6 to about 1.9 in visible range. Since a refractive index of Indium Tin Oxide (ITO) typically used as an anode ranges from about 1.9 to about 2.0, almost no total reflection between an organic light emitting layer and an anode occurs, so that no specific issue is raised. However, when the refractive index of a glass or plastic transparent substrate is about 1.5 and an organic light emitting layer and an anode layer of an organic light emitting diode has a very thin thickness of about 100 nm to about 400 nm, most of light generated from the organic light emitting layer becomes in a guided mode, thereby not being emitted to a device outside. The reason is that most of light generated from the organic light emitting layer is not emitted perpendicular to a substrate surface, and is incident to a substrate having a low refractive index at an angle almost parallel to the substrate surface. Therefore, in a typical organic light emitting diode, a ratio of light {circle around (4)} emitted to the outside of a glass substrate is very small, i.e. about 20% of the total amount of light emitted.
As known from Equation 1, if the refractive indices of materials at both sides of an interface are the same, an incident angle becomes identical to a refraction angle, so that no total reflection occurs. That is, when the refractive indices of an organic light emitting layer and an anode are identical or similar to the refractive index of a substrate, guided mode is minimized at the interface between the substrate and the anode. As a result, light extraction efficiency is increased, and power efficiency of an organic light emitting diode is increased also.
However, since the refractive index of ITO typically used as an anode is between about 1.9 to about 2.0, it is very difficult to find a substrate material having the same refractive index as an anode. Furthermore, since an organic light emitting diode emits light in an anode direction, a substrate material is required to have a very high transmittance in a visible region. However, a transparent substrate material having a refractive index of about 1.9 to about 2.0 and adequate strength and surface flatness as a substrate is very rare. Even if there is such a substrate material, it is very hard to manufacture a thin and flat large-sized glass by using the substrate material.
Accordingly, in order to manufacture a highly-efficient organic light emitting device, a method of improving light extraction efficiency needs to be provided and limitations derived from the above method need to be resolved.